Nickel has recently been shown to be a critical component of several enzymes which sustain certain life processes. For example, in hydrogenase (H2-ase), Ni is intimately involved in promoting electron-transfer to or from H2. When operating in concert with other enzymes, Ni H2-ase ultimately serves to interconvert electrochemical and chemical energy - a fundamental process in nature. The focus of the proposed research will be to develop an increased understanding of the role played by Ni in the metalloenzyme H2-ase by attempting to elucidate the relationship between structure and function. Critical questions to be answered concern the ability of the protein environment to stabilize Ni(III), which is an unusual oxidation state for Ni, and the role that Ni plays in promoting the reactions catalyzed by H2- ase. The proposal outlines strategies used in the design of a series of synthetic mononuclear Ni complexes which contain Ni in a sulfur-rich "binding pocket" resembling that of H2-ase. Our approach is unique in that we have designed our molecules to promote reactivity. By systematically varying the structure of our model complexes and probing the effect that this has on (1) redox behavior and (2) reactivity, we should be able to identify the environment(s) with is(are) best suited for the stabilization of Ni(III) and promoting reactivity with H2.